Positive train control (PTC) systems are currently under development in the United States and elsewhere. One benefit of a PTC system is to shorten the headways between successive trains on the same track segment, which can permit more traffic routing and traffic flow flexibility in planning and scheduling. In a PTC system, positive knowledge of the location of the end of the train is required since trains must maintain positive length of train awareness. Without end of train knowledge, the use of track occupancy circuits has to be maintained and/or their densities increased to support the current traffic density.
Accurate knowledge of the actual physical location of the rear end of a train is difficult to obtain because trains can vary in length during operation depending on whether the train is traveling on a descending grade, on an ascending grade, or at level grade. The length can vary as a result of the slack in couplers used to couple the cars to one another. Because of this ambiguity in train length, trains are typically managed by assigning each train a “safe train length” 2 which is longer than the actual train length 4 as shown in FIG. 1.
One example of a problem associated with assigning a safe train length 2 is illustrated in FIG. 1. A main railway track 6 may have a sidetrack or siding 8. A train 10 may be traveling on the track 6 in the direction indicated by the arrow. A second train (not shown) may be located on the sidetrack 8 waiting to enter onto the track 6 in the other direction. The second train must wait until an end of train car 12 of the train 10 passes the junction 14 before entering the track 6. With the assigned safe train length 2, the train 10 is assumed to be much longer than it actually is as indicated by the broken lines in FIG. 1. Therefore, the train waiting on the sidetrack 8 must wait until a trailing end Es of an assumed end of train car 12′ passes the junction 14, even though an actual trailing end Ea of the actual end of train car 12 of the train 10 has already passed the junction 14. As a result, entry of the train on the sidetrack 8 onto the track 6 is unnecessarily delayed.